Code Assignment in HS-SCCH Less Operation Mode

ABSTRACT

In a method and a system a good spread of users per code is obtained by providing a value related to the current load for each code in the system. The estimated load value can be used by the network to assign the HS-PDSCH code for users in HS-SCCH less operation, and may also be used in the dynamic transmission process, i.e. which codes are to be used by which user during this and future TTIs. If one code is over a given utilization threshold, where the threshold is set in order to avoid code blocking of VoIP users, enough users can be re-assigned to a different code. In the case that all of the current codes used in the HS-SCCH less operation model are over the utilization threshold, yet another code can be made available for HS-SCCH less operation.

TECHNICAL FIELD

The present invention relates to a method and a system for assigningcodes. In particular the present invention relates to a method and asystem for assigning codes in a cellular radio system employing a HighSpeed-Shared Control Channel (HS-SCCH) less mode of operation

BACKGROUND

Today mobile broadband services over cellular systems are becoming moreand more common. One underlying reason is the introduction of High SpeedPacket Access (HSPA) radio bearers in live networks. There is also anincreased interest in IP Multimedia Subsystem (IMS) based services, suchas Voice over IP (VoIP), and Push to talk over Cellular (PoC).

HSPA introduces the possibility of downloading and uploading data with aspeed of several Mbits/s, but there is also standardization work ongoingin 3GPP to boost VoIP capacity.

In High Speed Downlink Packet Access (HSDPA) a shared channel isemployed. The use of a shared channel results in that severalchannelization codes are shared between users on a per 2 ms TTI basisfor transmission.

In HSDPA the basic shared channel structure includes a number of codes,for example 8, which are available for High Speed-Downlink SharedChannel (HS-DSCH) transmission every 2 ms TTI. A user can use all codes,such as all 8 if 8 codes are available, or the codes can be dividedbetween users during the 2 ms Transmission Time Interval (TTI). Dividingcodes between different users is usually referred to as codemultiplexing, i.e. the users are multiplexed on the same TTI by beingassigned different codes. The UMTS Terrestrial Radio Access Network(UTRAN) assigns a number of High Speed Shared Control Channels(HS-SCCHs) to match the number of code multiplexed users.

Although a mobile station is only required to be able to listen to amaximum of 4 HS-SCCHs more than 4 HS-SCCHs can be deployed per cell. TheHS-SCCH carries information about which channelization codes that amobile station is to decode, the User Equipment (UE) identity, whichidentifies the receiver of the in formation, Hybrid Automatic RepeatRequest (HARQ) parameters and Transport Format and Resource Combination(TFRC) i.e. modulation scheme, channelization code set and transportblock size, etc.

Studies show that in order to allocate many VoIP calls in a single cellmore than 4 HS-SCCHs must be used. By allocating more than 4 HS-SCCHsper cell and spread out the UEs on these in a smart way higher codemultiplexing than 4 can be used. However, as the VoIP service iscomparably low rate service the HS-SCCH overhead may be high. Thereforea study item in 3GPP called continuous packet connectivity (CPC)investigated several schemes to improve services like VoIP, with focuson reducing overhead, see [3GPP TR25.903].

One improvement was the so called HS-SCCH less operation mode.Basically, in HS-SCCH less mode, the HS-SCCH overhead can be reduced bysimply being removed or introducing discontinuous transmission. Thedisadvantage is that only a few transport format combinations (TFC).i.e. packet sizes, can be used which are semi-static and configurableper UE. The UE makes a so called blind decoding assuming the configuredpacket sizes. In the currently proposed standard, two (2) differentTransport Block (TB) sizes are allowed in HS-SCCH less operation, andfour (4) different TB sizes are allowed in Reduced ComplexityHS-SCCH-less operation. If other TFCs or TB sizes are needed than thesetwo or four, then the normal High Speed Dedicated Physical ControlChannel HS-DPCCH must be used. For VoIP this is not a criticallimitation.

In normal operation mode, the network receives feedback information suchas channel quality indications and ACK/NACKs, on the uplink channelHS-DPCCH. The network utilizes that information in the downlinkscheduling decision and in the HARQ process. The downlink HS-SCCHindicates to the user which HS-DPSCH it shall decode, the HARQ processnumber and a CRC. Both parts employ a terminal specific masking, whichis used by the mobile station/terminal to determine that the data isactually intended for it. Finally the mobile station/terminal despreadsthe data sent on the HS-PDSCHs.

If HS-SCCH less operation is used, each VoIP user in HS-SCCH lessoperation is assigned a specific code, or HS-PDSCH channel, at the startof the session. Hence, the code assignment is highly important, and careshould be taken to avoid code blocking as the mobile stations in HS-SCCHless mode only try blind decoding on the given pre-defined HS-PDSCHcode, i.e. channelization code. It is of course possible to change thiscode, but not instantaneously since it requires an RRC message, see 3GPPTS 25.308.

Code blocking is when the scheduler assigns two or more users totransmit in the same TTI, but since they use the same HS-PDSCH code thisis not possible. In normal operation the code assignment is done on thefly by using the HS-SCCH to point out the HS-PDSCH code.

Today there exist no solution to handle this situation. One solutioncould be to assign x out of C possible HS-PDSCH codes, for all VoIPusers, and all other codes (C-x) for other (non CPC) services, such asweb browsing, etc. It should be noted that the network cannot transmitduring the same TTI to users in HS-SCCH less operation which areassigned the same code.

However, when the number of VoIP users increases in the cell such asolution is less efficient. When the number of VoIP users increases anddominates the traffic in cell, the probability to simultaneouslytransmit to many VoIP users increases. And since the code limit isfixed, there is always a chance that the need for codes exceeds thatcode limit. Without HS-SCCH less operation and CPC. 4 users per TTI wastypically the code limit. This since the standard stipulated that eachUE only listens up to 4 groups of codes (HS-SCCH channels). Furthermore,it is not obvious how to spread the users among the codes available.

Hence, there exist a need for a method and a system that is able tohandle many VoIP calls in a single cell.

SUMMARY

It is an object of the present invention to overcome or at least reducesome of the problems associated with set up of many VoIP calls in a cellof a cellular radio system.

It is another object of the present invention to provide a system and amethod that enables efficient assignment of codes to mobile stations inorder to achieve efficient transmission and capacity.

These objects and others are obtained by the method and system as setout in the appended claims. Thus, by providing a good spread of usersper code, the VoIP capacity limit can be increased and maximized. Thissince it is not possible to transmit to two users with the same codeduring one TTI. Instead a traffic load value, such as an intensityvalue, is estimated per user. As input to the estimation function avalue related to the current load in the system can be used.

For example input to the estimation function may include parameters suchas, MAC-hs buffer level, packet arrival time, number of retransmissions,etc. The estimated intensity value can be used by the network to assignthe HS-PDSCH code for users in HS-SCCH less operation, and may also beused in the dynamic transmission process, i.e. which codes are to beused by which user during this and future TTIs.

If one code is over a given utilization threshold, where the thresholdis set in order to avoid code blocking of VoIP users, enough users canbe re-assigned to a different code. In the case that all of the currentcodes used in the HS-SCCH less operation model are over the utilizationthreshold, yet another code is preferably made available for HS-SCCHless operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way ofnon-limiting examples and with reference to the accompanying drawings,in which:

FIG. 1 is a view illustrating a cellular radio network where a number ofmobile stations are enabled to communicate using a HS-SCCH lessoperation mode.

FIGS. 2 a and 2 b are illustrations of a switching scheme betweendifferent operation modes in a cellular radio system, and

FIG. 3 is a flow chart illustrating a procedure for assigning codes in acellular radio system.

DETAILED DESCRIPTION

In FIG. 1 a view illustrating a part of an exemplary cellular radiosystem 100 is shown. The system 100 is enabled to operate in a HS-SCCHless mode. The system 100 comprises a number of base stations, NodeBs,101 whereof one is depicted in FIG. 1. Each base station 101 can be usedto establish radio communication with a number of mobile stations 103,User equipments UE, camping with in the coverage area of a base station.The base station 101 comprises a module 105 for assigning codes to themobile stations and also the base station 101 comprises a unit 107 forestimating the load of in the cell serviced by the base station as isdescribed more in detail below.

In normal, non-HS-SCCH less, operation, the maximum number of HS-PDSCHcodes used by the base station 101 of FIG. 1 are for example equallydivided by the (maximum four) users, i.e. the code assignment is done onthe fly. However, this is not possible for users using HS-SCCH lessoperation. Therefore, it is important to carefully assign the codes tothe HS-SCCH less users so that code blocking does not occur between theHS-SCCH less users or between other users.

Hence, optimally the HS-SCCH less users should be assigned codes by thebase station 101 in such a way that during high load situations thereare no holes/gaps in the time/code structure.

To achieve a transmission pattern where no are no holes/gaps in thetime/code structure exists an estimation algorithm/function as describedbelow can be used. Thus in order to evenly spread the HS-SCCH lessusers, the system is provided with the unit 107 for estimating the load,i.e. the relative time the service needs to transmit in the down link.

For example take VoIP using an Adaptive Multi Rate (AMR) codec as anexample assume a certain activity rate λ_(s), e.g. a packet will betransmitted every 20 ms (activity rate of 1/10). By summing the totalactivity rate for a certain service (in our example VoIP), a measure ofhow often the service will utilize the down link channel is acquired. Aslong as the sum is lower than a certain threshold the same HS-PDSCH canbe used. When the threshold is exceeded, a new HS-PDSCH code is assignedto the new users.

In a preferred embodiment the service load per code, taking VoIP asexample, is based on one or more of the following parameters:

The number of conversational (VoIP) RABsThe packet arrivals per TTI

Retransmissions

Buffer levels

Additionally radio resources/conditions such as transmitter power andsignal strengths can be taken into account.

The HS-SCCH mobiles are assigned a HS-PDSCH code according to the totalservice load above. Further, the HS-PDSCH code assignment is preferablycontinuously updated for the codes to be spread evenly, since some userswill hang up or be inactive for a longer period.

In FIG. 2 a a flow chart illustrating steps performed when switchingbetween a normal mode and a HS-SCCH less mode in a base station isshown. Thus, if the system is handling users in a normal mode ofoperation and the maximum number of codes or other radio resource suchas power and signaling overhead required to maintain operation is belowa first threshold value, Threshold1, as determined by comparing a firstfunction with the first threshold value, the operation can continue in anormal mode of operation. If however it is determined by comparing saidfirst function of existing users with the first threshold that at leastsome of the users need to be transferred to a HS-SCCH less mode ofoperation at least some of the users are transferred to the HS-SCCH lessmode.

Similarly in FIG. 2 b a flow chart illustrating steps performed whenswitching between a HS-SCCH less mode and a normal mode in a basestation is shown. Thus, if the system is handling users in a HS-SCCHless mode of operation and the maximum number of codes or other radioresource such as power and signaling overhead required to maintainoperation is above a second threshold value, Threshold2, as determinedby comparing a second function with the second threshold value, theoperation will continue in HS-SCCH less mode of operation. If however itis determined by comparing said second function of existing users withthe second threshold that at least some of the users can be transferredback to a normal mode of operation at least some of the users may betransferred to the normal mode. The first and second function andthresholds used for determining switching between the two modes may butdon't have to be the same functions and thresholds, respectively.

In FIG. 3, a flow chart illustrating a procedure for assignment of codesin a system supporting a HS-SCCH less transmission mode for users in aHS-SCCH less mode of operation. First in a step 301 the load generatedby each user is determined. The load can for example be determined usinga function of a number of parameters indicative of the load such aspacket arrival rate, number of re-transmission, buffer level, etc. Nextin a step 303 for each of the available codes the combined load by allusers assigned to that particular code is determined.

Next, in a step 305, each new user is assigned to a code which satisfiesthe condition that the estimated additional load added by the new userand the existing load of that code together is below a threshold valuefor that particular code. Next, in a step 307 the procedure continuouslyor periodically checks the load for each code against the determinedthreshold for each code. If the load for a particular code is exceeded,one or more users using that particular code is re-assigned to anothercode. If no code is available to transfer such users to a new code isgenerated.

In equation form this can be expressed as follows;

$\begin{matrix}{{L_{x} = {\sum\limits_{n}^{K}S_{n}}}{L_{x}\text{:}\mspace{14mu} {Load}\mspace{14mu} {on}\mspace{14mu} {code}\mspace{14mu} x}{S_{n}\text{:}\mspace{14mu} {Service}\mspace{14mu} n}{K\text{:}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {services}}} & \left\lbrack {{eq}\mspace{14mu} 1} \right\rbrack \\{{S_{n} = {\sum\limits_{i}^{N}\gamma_{n,i}}}{\gamma_{n,i}\text{:}\mspace{14mu} {load}\mspace{14mu} {of}\mspace{14mu} {service}_{n}\mspace{14mu} {by}\mspace{14mu} {user}_{i}}{N\text{:}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {users}}} & \left\lbrack {{eq}\mspace{14mu} 2} \right\rbrack \\{{\gamma_{n} = {f\left( {\lambda,{rtx},{bl}} \right)}}{f\text{:}\mspace{14mu} {function}}{\lambda \text{:}\mspace{14mu} {packet}\mspace{14mu} {arrival}\mspace{14mu} {rate}}{{rtx}\text{:}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {retransmissions}}{{bl}\text{:}\mspace{14mu} {buffer}\mspace{14mu} {level}}} & \left\lbrack {{eq}\mspace{14mu} 3} \right\rbrack\end{matrix}$

The function in equation 3 may simply be the packet arrival rate plus afiltered retransmission value, for example as a mean value that couldapply to the buffer level.

If the load is larger than a threshold value, the network is adapted tore-assign HS-SCCH users to a different code. In accordance with oneembodiment, the code multiplexing can be increased if all codes havehigher load than the given threshold. This is illustrated in equation 4.

iƒL _(x) >Th, then assign user_(i) to L _(y) where L _(y)+γ_(n,i)≦Th  [eq 4]

Equation 3 gave three input parameters to estimate the contribution tothe load value of a user. However, other input parameters such as powerusage and interference levels and similar may be added to the function.The threshold value of equation 4 may also be used to accomplishdifferent objectives such as;

equal power usage per codeequal share of traffic per code

Using the method and system as described herein will avoid that system(VoIP) capacity and the capacity of similar services is unnecessarilylimited due to code shortage. Further, the interference spread per userwill be decreased.

1-16. (canceled)
 17. A method of assigning codes for users in a cellularradio system operating in a High Speed Shared Control Channel (HS-SCCH)less mode of operation, the method comprising: determining the systemload generated by a user communicating with the system in a HS-SCCH lessoperation mode; and assigning a code for the user in response to saiddetermined system load.
 18. The method of claim 17, wherein assigning acode for the user comprises assigning a High Speed Physical DownlinkShared Channel (HS-PDSCH) code.
 19. The method of claim 17, whereindetermining the system load generated by a user comprises determiningthe system load based on one or many of the following parameters: MAC-hsbuffer level; packet arrival time; number of re-transmissions; transmitpower usage; and interference level.
 20. The method claim 17, furthercomprising: determining if one used code is above a predeterminedutilization threshold value.
 21. The method of claim 20, wherein, ifsaid one used code is determined to be above the predeterminedutilization threshold value, one or more users assigned to that code isreassigned to another code.
 22. The method of claim 20, furthercomprising: making another code available if all used codes exceed theirrespective predetermined utilization threshold values.
 23. The method ofclaim 17, further comprising: continuously or periodically determiningthe load caused by all users of a particular code and comparing saidcombined load to a pre-determined utilization threshold value.
 24. Themethod of claim 23, wherein, if a used code is determined to be above apredetermined utilization threshold value, one or more users assigned tothat code is reassigned to another code.
 25. A node of a cellular radiosystem operating in a High Speed Shared Control Channel (HS-SCCH) lessmode of operation, the node adapted to assign codes for users andcomprising: a load determination circuit configured to determine thesystem load generated by a user communicating with the system in aHS-SCCH less operation mode, and a code assignment circuit configured toassign a code for the user in response to said determined system load.26. The node of claim 25, wherein the code assignment circuit isconfigured to assign a High Speed Physical Downlink Shared Channel(HS-PDSCH) code.
 27. The node of claim 25, wherein the loaddetermination circuit is configured to determine the system load basedon one or many of the following parameters: MAC-hs buffer level; packetarrival time; number of re-transmissions; transmit power usage; andinterference level.
 28. The node of claim 25, wherein the loaddetermination circuit is further configured to determine if one usedcode is above a predetermined utilization threshold value.
 29. The nodeof claim 28, wherein the code assignment circuit is configured tore-assign one or more users assigned to said one used code to anothercode, if said one used code is determined to be above the predeterminedutilization threshold value.
 30. The node of claim 28, wherein the codeassignment circuit is configured to make another code available if allused codes exceed their respective predetermined utilization thresholdvalues.
 31. The node of claim 25, wherein the load determination circuitis configured to continuously or periodically determine the load causedby all users of a particular code and compare said combined load to apre-determined threshold value.
 32. The node of claim 31, wherein thecode assignment circuit is configured to re-assign one or more usersassigned to a code to another code, if that code is determined to beabove a pre-determined utilization threshold value.